1. Field of the Invention
The present invention relates to a valve opening and closing mechanism which is preferable to be used, for example, for opening and closing an operating fluid passageway in a wheel cylinder for expanding brake shoes of a drum brake device.
2. Description of the Related Art
Conventionally, various types of drum brake devices are used for braking the running of vehicles. These drum brake devices are classified into a leading trailing type, a two leading type, a duo-servo type, and so on, in accordance with the arrangement of brake shoes pressed against an inner circumferential surface of a substantially cylindrical drum.
A duo-servo type drum brake device is generally provided with a pair of brake shoes, which are primary and secondary shoes disposed oppositely to each other in a cylindrical drum.
The primary shoe is designed so that its entrance side in the forward rotation direction of the drum is served as an input portion while its exit side in the forward rotation direction of the drum is, for example, connected to the entrance side of the secondary shoe through an adjuster. On the other hand, the exit side of the secondary shoe is made to abut against an anchor portion mounted on a backing plate so as to receive anchor reaction force acting on the primary and secondary shoes.
Consequently, when the primary and secondary shoes are expanded and pressed against the inner circumferential surface of the drum, the anchor reaction force acting on the primary shoe is put into the entrance side of the secondary shoe. Thus, the anchor reaction force acts on the secondary shoe so as to press the secondary shoe against the inner circumferential surface of the drum. As a result, both the primary and secondary shoes act as leading shoes so that braking force with very high gain can be obtained.
The above-mentioned duo-servo type drum brake device has many advantages in comparison with the leading trailing type or two leading type drum brake device in that not only it can gain an extremely high braking force but also it can be miniaturized easily, and it can be incorporated with a parking brake easily.
Such a duo-servo type drum brake device is, however, sensitive to a change in the friction coefficient of the lining of each brake shoe. Accordingly, the duo-servo type drum brake device tends to be difficult to stabilize its braking force. A device for stabilizing the braking force is therefore needed.
Thus, the applicant of this application has made various proposals about a wheel cylinder which can control pressing force transferred to brake shoes, so that anchor reaction force is limited to a predetermined ratio relative to the fluid pressure supplied from a master cylinder in order to stabilize the braking force in a duo-servo type drum braking system. Further, the applicant of this application has an object to develop a high-performance wheel cylinder which can perform relaxation of impact at the time of quick braking and fine control of braking force.
To develop such a wheel cylinder, it is necessary and essential to develop a valve for precisely and smoothly controlling the supply of operating fluid supplied from a master cylinder. Such a valve has been therefore regarded as one of most important problems to be solved in the future.